Means and method for detection of glaucoma

ABSTRACT

The acoustic impedance of the cornea is measured by comparing the energy it reflects with the energy reflected by a calibrated, variable impedance member.

United States Patent Lichtenstein et al.

[ Sept. 12, 1972 [54] MEANS AND METHOD FOR DETECTION OF GLAUCOMA [72]Inventors: Bernard Lichtenstein, 1825 Highbrook SL, Yorktown Heights,NY. 10578; Bruce G. Kroger, Purchase 81., Rye, N.(. 10580 [52] US. Cl..73/80, 73/678 [51] Int. Cl. ..G0ln 29/00, A61b 3/16 [58] Field ofSearch ..73/67.l, 67.8, 80; 128/2 [56] References Cited UNITED STATESPATENTS 2,394,461 2/1946 Mason........................73/67.1

2,618,968 11/1952 McConnell ..73/67.8 X 3,371,660 3/1968Carlin............................128/2 OTHER PUBLICATIONS Mundt et a1Ultrasonics in Ocular Diagnosis" American Journal of Opthalmology Vol.41, No. 3, March 1956, 488-98. RE 1 A5 in Library Copy in 73/80.

Primary ExaminerRichard C. Queisser Assistant Examiner-C. E. Snee, IIIAttorney-S. P. Tedesco and S. E. Rockwell ABSTRACT The acousticimpedance of the cornea is measured by comparing the energy it reflectswith the energy reflected by a calibrated, variable impedance member.

2 Clairm, 1 Drawing Figure MEANS AND METHOD FOR DETECTION OF GLAUCOMAThis patent application is related as a divisional application to US.Pat. application Ser. No. 679,379, filed on Oct. 31, 1967 (now US. Pat.No. 3,545,260).

BACKGROUND OF THE INVENTION 1 Field of the Invention This inventionrelates to the measurement of pressure within a closed container, andparticularly to the measurement of pressure within an invertebrate eye.

2. Description of the Prior Art The determination of the liquid pressurein the anterior eye chamber (intraocular pressure) is of great interestin the diagnosis of glaucoma. While the causes of glaucoma are notcompletely understood, its diagnosis and treatment are fairly wellunderstood. Glaucoma is a malfunction of the eye mechanism which resultsin impairment of the circulation of the aqueous humor through the eye,and thereby leads to a build-up of liquid pressure within the eye. Thispressure build-up, if permitted to persist, can damage the optic cup andthe retina, eventuating in blindness.

The relationship between the pressures in the normal and in theglaucomatous eye has been known since 1830. The formation of the aqueoushumor and its circulation within the eye was subsequently theorized andproven in 1837 by the injection of a dilute dye into the anteriorchamber of the eye and then noting the presence of this dye in theepiscleral vessels.

Various methods and apparatuses have been developed to accomplish"tonometry" or the observation and the recording of pressure changes inany physical or biological system, as discussed by P.C. Kronfield inTransactions of the American Academy of Ophtalmology and()tolaryngology, l33:Mar/April 1961 Subsequently the massage effect" wasdiscovered, wherein external pressure applied to the eye causes a dropin intraocular pressure. J. S. Friedenwald in I, Am. J. Ophtalmology20:985 1937; II, ibid, 22:375, 1939, published a table relating thevolume of corneal indentation and intraocular pressure to the plungerloads and scale readings of the "Schiotz tonometer. There are severaltypes of tonometers in common use. They are the Schiotz, the Gradle, theMcLean, the Bailliort, the Sunter and the Harrington. All except theBailliort are used in a similar manner in that the tonometer is handsupported and the footplate, which is weighted, is allowed to contactthe cornea. The Bailliort tonometer footplate contacts the temporalsclera about mm. from the limbus, instead of the cornea. ln use, contactis made to correspond to an initial Schiotz scale reading and thesubsequent fall of the plunger is followed by the pivoted lever andmeasured by the deflection of the lever against an upper scale. Duringthis measurement the subject must be in a reclining position with thecornea anesthetized.

Friedenwalds tables relate the initial tonometer reading to steady stateor intraocular pressure (P,) by the following equation:

where AV is the net ocular displacement; K, is the coefticient of ocularrigidity with respect to ocular distortion; and P is the intraocularpressure as a result of applying the tonometer.

The values of V are determined to be a function of the tonometer depth.P, is calculated from the calibrated plunger weight and diameter. Thereis no single intraocular pressure level that demarks a healthy eye froma glaucomatous eye. The normal variations of intraocular pressure arebetween 16 and 25 mm. Hg. The average intraocular pressure reading takenin 1,000 eyes of patients over 30 years of age was reported as 19,63mm.l-Ig. by Berens and Zuckerman in Diagnostic Examination of the Eye,1946. This changes with age, being lowest between 30 and 40 years,highest between 60 and years, and decreasing after 70. Intraocularpressure above 25 mm. Hg. is considered suspicious, and above 28,pathologic. G. W. Morton demonstrated a method for measuring the"Coefficient of Aqueous Humor Outflow" or (C) in AMA Archives ofOphthalmology 46:1 l3, 1951. This coefficient, with the intraocularpressure, enables the calculation of the total flow of aqueous humor.The assumption is made that the change in ocular volume (AV is equal tothe volumes of fluid expressed from the eye, and that the eye acts as alinear mechanical system. Thus, where (AP) is the change in pressureduring tonometry, as measured by a continuous reading tonometer over afixed interval of time (T), and (P,,) is the initial intraocularpressure,

then

AV -AP 01' AV CAP (2 and C being the coefficient of facility of aqueousoutflow. Since all terms on the right of equation are measured by thecontinuous reading tonometer, C can be calculated.

Since Mortons work in 1950, a great many measurements of C have beenmade which have indicated the diagnostic value of the coefficient offacility of aqueous humor outflow. The glaucomatous eye tends to have alow C and a high P.,. Values of C greater than 0.18 are normal, valuesbetween 0.13 and 0.18 are suspect, and values less than 0.13 areprobable glaucoma. For subjects in the 0.13 to 0.18 range, the ratio ofintroocular pressure to coefficient of facility of outflow (P /C) isuseful, setting the norm at a value of 100. P,,/C values of greater thanare considered normal, and less than 100 are suspect.

Tabulations of C and P, require a knowledge of the coefficient of ocularrigidity (K,). The values used are empirically determined mean, and canresult in errors in C and P, when eyes with abnormal ocular rigidity areexamined. In view of this, it is desirable to measure P, whileintroducing a minimum of surface and fluid distortion to the eye. In1956, H. Goldman devised the Applanation Tonometer, Trans. Ophthal, Soc.U.l(. 79:477, 1959, wherein pressure application is carefully controlledand limited to the amount required to just flatten a small surface ofthe cornea with a 3.06 mm. diameter footplate, which createsapproximately a 200 micrometer corneal indentation. Values of P,obtained this way have proven more reliable, and the combination ofvalues of P, obtained by applanation, and C obtained by tonography arethe most reliable glaucoma diagnostic tools commonly available.

SUMMARY OF THE INVENTION Objects of this invention are to provide amethod and apparatus for tonometry utilizing a scleral distortion muchsmaller than that previously feasible, e.g., 2.5 micrometer indentation,and to make possible a measurement of P, with more accuracy and lesseffect on the eye. Other objects are to provide a technique which doesnot require a topical anesthetic, nor dyes applied to the eye; to allowmeasurement in complete comfort to the subject without mechanicalcontact with the cornew, and to provide a measurement which can be madesimply, swiftly, and to both eyes concurrently if desired.

A principle of the invention is a system wherein the acoustic impedanceof the eye is measured by ensonifying and comparing the eye acousticimpedance with a known acoustic impedance.

BRIEF DESCRIPTION OF THE DRAWING These and other objects, features andadvantages of the invention will be apparent from the followingspecification thereof, taken in conjunction with the accompanyingdrawing, in which:

The FIGURE is a block diagram of an apparatus for measuring theintraocular pressure by measuring the acoustic impedance of the anteriorchamber.

DESCRIPTION OF THE PREFERRED EMBODIMENT Radiated energy, of any type, beit acoustic, ultrasonic, or electromagnetic, will experience reflectionand refraction at boundaries of change in the propagating medium. In thecase of a propagated acoustic wave, the amount of reflected energy is afunction of acoustic impedance mismatch at the boundary. The relativeintensity of reflected to propagated energy for normal incidence isdefined by:

Where, I,- intensity reflection coefiicient,

Z 1 Acoustic impedance of propogation source,

Z, Acoustic impedance of propogation incidence.

Echo intensity is also a function of the angle of the incident energywith respect to the incident boundary and the distance from thetransducer to that boundary. The echo intensity is very sensitive to theangle of incidence, changing as much as 40 percent for a 1 inclinationchange at 18 mos. Changing the transducerto-boundary spacing by threewavelengths at 18 mcs. can produce a 20 percent variation.

If the spacing is held constant and the angle of incidence is carefullycontrolled, then the coefficient of reflection varies in response to theacoustic mismatch. Then, if the acoustic impedance of a first medium isknown, the acoustic impedance of a second medium can be determined.

It appears that the intraocular pressure of the eye is responsive to theacoustic impedance of the cornea,

and this relationship is constant. A system for measuring the acousticimpedance of the cornea is shown in the FIG. 1. In this system the eyeis ensonified by bursts of high frequency acoustic energy and the echolevel is visually displayed in the face of the cathode ray tube.Simultaneously, the same energy is used to ensonify, through a likeinterface material, a material whose acoustic impedance Z, iscalibrated. The second echo so produced is displayed above the firstecho on the same tube face. By selecting the calibrated impedance Z,until the echo intensities are equal, a value of energy is reflectedwhich is equal to that of the ensonified portion of the eye. To effectsuch selection, a number of containers, such as 106-110 and eachincluding a transducer 108, are operatively substituted, in turn, intothe system, so as to be connected to the transmitreceive switch 120 byan appropriate switching arrangement, not shown. The containers 106 ofsuch arrangements include different calibrated acoustic impedances, Z,which are used as standards for purposes of comparison to determine theacoustical impedance of the cornea.

The subject disposes his eye in a container so that the distance d ofthe cornea from the right transducer 102 and the left transducer 104 isequal to the distance d of a body 106, whose acoustic impedance iscalibrated, from a transducer 108 in a container 110. Both of thecontainers are filled with a fluid whose acoustic impedance approximatesthat of the eye.

A clock pulse generator 112 generates a fixed sequence of pulses whichform the time base for the system. Each clock pulse enables atransmitter driver 114 which drives a pulse transmitter to feed a highfrequency pulse to two transmit-receive switches 118 and 120. The clockpulses from the generator 112 are also coupled to a logic network 122which alternately enables switch 118 and switch 120. Switch 118, whenenabled, transmits the high frequency transmitter pulses to either theright or lefi transducers 102 or 104, as determined by a right or leftselect circuit 124, which propagates the energy through the mediumhaving an acoustic impedance Z, contained in the container 100 to therespective right or left cornea. The echo returns through the sametransducer to the switch 118, which is held on by the logic network 122.The echo is coupled through a receiver 126, amplified in a verticalamplifier 128, and coupled by an alternate vertical deflection circuit130 to the vertical input of the CRT 132 and displayed as a lower trace.The same clock pulse from the generator 112, which initiated thissequence, turns on a sweep generator 134, which starts the CRThorizontal trace via the sweep amplifier 136 which is coupled to thehorizontal input of the CRT. The initial pulse (I) displayed in the CRTis part of the transmit pulse which feeds through the receiver chain.The subsequent pulse (E) is the echo pulse. The sequence is repeated forthe next clock pulse from the generator 112 which causes the logicnetwork 122 to turn on the transmit-receive switch 120. This couples thehigh frequency pulse from the transmitter 116 to the transducer 108disposed in the standard container which is also filled with a mediumwhich is substantially the same as Z,. The burst of energy is reflectedfrom the body 106, and this echo is coupled through the receiver 126,the amplifier 128 and the alternate vertical deflection circuit to thevertical input of the CRT, and displayed as an upper trace. The tracesare alternated as described by the alternate deflection circuit whichadds a voltage increment to the pulses from the switch 120.

The impedance Z, of the body 106 is selected such that the echointensities are matched, thereby providing a known value of 2', equal tothe ensonified portion of the eye.

When the acoustic impedance of the interface material 2,, such as wateror light oil, is closely matched to the acoustic impedance of the eye,the coefficient of reflection intensity (1,) is very sensitive tochanges in the acoustic impedance of the eye (2,),

Thus,

Z Z,, 2 u* iZFZn I a: Z 1 Z;

Where, I Intensity of reflection coefficient for a normal eye; IIntensity of reflection coefficient for a glaucomatous eye;

Z 1 Acoustic impedance of the interfaces material; 2,, Acousticimpedance of the normal eye,

Z, Acoustic impedance of the glaucomatous eye. By letting A Z,,/Z,,, B=l,,,/l,,, D= Z,/Zn,

Thu D 1 2 Di: A 2

B r X [D -A which, when plotted, letting said B be the variable,provides a family of curves as a function of D. In these curves, theratio of coefficient of reflection changes very rapidly for smallchanges in acoustic impedance of the eye when the initial impedancematch is close to 1. This sensitivity descreases as the initial acousticimpedance match is degraded, e.g., D becomes smaller. When the initialacoustic impedance ratio is 0.9, a coefficient of intensity ofreflection ratio of 0.5 is obtained by the impedance ratio A changingapproximately 0.05. Thus, a 5 percent change in corneal acousticimpedance between a normal eye and a glaucomatous eye will result in a 6db change in reflected energy. This level change is a good minimumdetectable criteria.

it may be noted that acoustic impedance is the product of density andvelocity of sound. Assuming that the cornea does not stretch and thatthe sclera] tissue is totally compressible, then there is a linearrelationship between scleral density and intraocular pressure. in thiscase, velocity being constant, the acoustic impedance of the sclera isdirectly related to intraocular pressure. Thus, this system has thecapability of detecting a 5 percent change in intraocular pressure.

While there has been shown and described the preferred embodiments ofthe invention, it will be understood that the invention may be embodiedotherwise than as herein specifically illustrated or described, and thatcertain changes in the form and arrangement of parts and in the specificmanner of practicing the invention may be made without departing fromthe underlying idea or principles of this invention within the scope ofthe appended claims.

Y. iri efli d if determining the acoustic impedance of the cornea,comprising:

directing a first acoustic pulse through a first coupling medium, so asto be reflected back from the cornea, said medium having an acousticimpedance which closely approximates that of the eye,

measuring the reflected energy of said first acoustic pulse;

directing a second acoustic pulse, substantially identical to said firstacoustic pulse, through a second coupling medium having acousticimpedance substantially equal to that of said first coupling medium, soas to be reflected back from a calibrated acoustic impedance;

measuring the reflected energy of said second acoustic pulse;

determining the respective paths of said first and second acousticpulses and the reflections thereof through said first and secondcoupling mediums, respectively, to be substantially identical;

detecting and comparing the respective reflected energies of said firstacoustic pulse and said second acoustic pulse; and

selecting said calibrated acoustic impedance to provide that thereflected energy of said second acoustic pulse is equal to the reflectedenergy of said first acoustic pulse, so as to determine the acousticimpedance of the cornea by comparison.

2. Apparatus for determining the acoustic impedance of the corneacomprising:

a cup-like member having its open end disposed in sealed relationship tothe cornea and containing a first liquid medium therein having anacoustic impedance which approximates that of the eye;

means including a first transducer in said cup-like member for directingacoustic energy through said first liquid medium against said cornea andreceiving reflected energy therefrom as a function of the acousticimpedance of the cornea;

means including a second transducer disposed in sealed relationship to acalibrated acoustic impedance in a second liquid medium having anacoustic impedance substantially equal to that of said first liquidmedium and spaced from said calibrated impedance a distancesubstantially equal to the distance between said first transducer andsaid cornea, said second transducer being operative to direct a sameacoustic energy through said liquid medium against said calibratedimpedance, said calibrated impedance being such as to reflect a sameenergy therefrom to said second transducer as is received by said firsttransducer; and

means for comparing the energy reflected from said calibrated impedanceand that reflected from said cornea to said second and firsttransducers, respectively, whereby the acoustical impedance of thecornea is determined by comparison.

1. A method for determining the acoustic impedance of the cornea,comprising: directing a first acoustic pulse through a first couplingmedium, so as to be reflected back from the cornea, said medium havingan acoustic impedance which closely approximates that of the eye,measuring the reflected energy of said first acoustic pulse; directing asecond acoustic pulse, substantially identical to said first acousticpulse, through a second coupling medium having acoustic impedancesubstantially equal to that of said first coupling medium, so as to bereflected back from a calibrated acoustic impedance; measuring thereflected energy of said second acoustic pulse; determining therespective paths of said first and second acoustic pulses and thereflections thereof through said first and second coupling mediums,respectively, to be substantially identical; detecting and comparing therespective reflected energies of said first acoustic pulse and saidsecond acoustic pulse; and selecting said calibrated acoustic impedanceto provide that the reflected energy of said second acoustic pulse isequal to the reflected energy of said first acoustic pulse, so as todetermine the acoustic impedance of the cornea by comparison. 2.Apparatus for determining the acoustic impedance of the corneacomprising: a cup-like member having its open end disposed in sealedrelationship to the cornea and containing a first liquid medium thereinhaving an acoustic impedance which approximates that of the eye; meansincluding a first transducer in said cup-like member for directingacoustic energy through said first liquid medium against said cornea andreceiving reflected energy therefrom as a function of the acousticimpedance of the cornea; means including a second transducer disposed insealed relationship to a calibrated acoustic impedance in a secondliquid medium having an acoustic impedance substantially equal to thatof said first liquid medium and spaced from said calibrated impedance adistance substantially equal to the distance between said firsttransducer and said cornea, said second transducer being operative todirect a same acoustic energy through said liquid medium against saidcalibrated impedance, said calibrated impedance being such as to reflecta same energy therefrom to said second transducer as is received by saidfirst transducer; and means for comparing the energy reflected from saidcalibrated impedance and that reflected from said cornea to said secondand first transducers, respectively, whereby the acoustical impedance ofthe cornea is determined by comparison.